Noun:
Someone who is really lazy.
usually describes someone who is chilling at home, wearing pijamas all day, being really slow with everything.

It's can be used as something negative OR positive depending on the situation.

If it happens once in a while and you just have a chilled out day at home with your friends, a schlomo day in front of TV can be awesome.

If you are just a lazy person, who eats to much, gets fat, never goes out, ditched his friends and just schlomes around all day, it obviously is negative.

To use it as an insult:
Even if you don't know the person, but you can suggest from his look that he is quite a schlomo (either he/she is fat, looks like a computer geek or looks totally boring), you can use the term as an insult. In this case schlomo has the same meaning as loser.

In this case you can use it instead of the word "loser".

M: "Fuck, I failed on my driving test for the third time. I'm such a schlomo."
L: "Oh jheezz, yesterday this schlomo from work made a move on me..how can he possibly think that I would go out with him?"

G: "Hey dude, I tried reaching you all weekend, there was a massive pool-party at my freinds place, what have you been up to?"
M: "I didn't do anything this weekend, I stayed at home in my PJ's watching O.C. all over again from season 1."
G: "Oh man, you are such a schlomo, you missed out on the party of the year!!!"